1. Field of the Invention
The present invention relates to a pattern drawing apparatus and pattern drawing method for forming patterns, that have a mirror image relationship to each other with respect to a substrate, on both sides of the substrate, and a test apparatus for use in the pattern drawing apparatus.
2. Description of the Related Art
When forming patterns, that have a mirror image relationship to each other with respect to a substrate, on both sides of the substrate, the patterns formed on the respective sides of the substrate must be made to exactly match in shape and position. That is, when the substrate is optically observed therethrough from one side, the patterns formed on the respective sides must appear to be superimposed.
FIG. 8 is a diagram illustrating one side of a substrate on both sides of which are formed patterns that are mirror images of each other. FIG. 9 is a diagram illustrating the other side of the substrate on both sides of which are formed the patterns that are mirror images of each other.
Representative examples of patterns that have a mirror image relationship to each other with respect to a substrate include lead patterns of a leadframe member. A leadframe is used in an IC package, such as a dual in-line package (DIP), to electrically connect the IC chip to external devices.
As described in Japanese Unexamined Patent Publication No. 5-190531, a leadframe is formed by etching or punching a nickel-plated Kovar or copper plate with a thickness of about 0.2 mm.
Further, as described in Japanese Unexamined Patent Publication Nos. 2-158160 and 2001-42544, when forming a lead pattern by etching, a resist pattern conforming with the desired lead pattern is formed on a leadframe member, and the leads are formed by applying etching.
FIG. 10 is a diagram illustrating alignment marks which are needed for accurately forming patterns that are mirror images of each other on both sides of a substrate. Generally, in a leadframe fabrication process using etching, alignment marks 60 such as shown in FIG. 10 are formed in advance at prescribed positions on both sides of the substrate as guide marks for accurately aligning the positions of the patterns between the two sides of the substrate to be exposed to radiation. In the figure, the alignment marks are greatly exaggerated relative to the size of the substrate, but actually, they are as small as pinholes.
Using the alignment marks as guide marks, a photoresist is applied over one side (hereinafter called the “side A”) of the substrate and, after placing a photo mask so as to form an exposure pattern on the side A, the side A is exposed to radiation.
Likewise, using the alignment marks as guide marks, a photoresist is applied over the other side (hereinafter called the “side B”) of the substrate and, after placing a photo mask so as to form an exposure pattern on the side B, the side B is exposed to radiation.
Here, the photo mask placed on the side B is in a mirror image relationship to the photo mask placed on the side A and, using the alignment marks formed on the sides A and B as guide marks, the position of the photo mask placed on the side B is adjusted so that the positions of the exposure patterns coincide with each other on both sides of the substrate. In this specification, this adjusting process is called the “photo mask alignment process.”
When exposure is completed on both the side A and side B of the substrate, the substrate is developed to form resist patterns, and portions exposed from the resist patterns are etched away to obtain a leadframe having the desired pattern.
In the prior art leadframe fabrication method using etching, as two photo masks of different patterns are required, one for each side of the substrate, the cost is relatively high. In particular, as the number of trials increases, the cost increases.
Furthermore, the prior art requires the photo mask alignment as described above. As for the shapes of the photo mask patterns, this can be accomplished relatively easily as it only requires mirror image conversion of data. However, for the positional alignment of the photo mask patterns, as the two sides of the substrate have to be optically treated using the alignment marks as guide marks, the fabrication process becomes complex and time-consuming.
Additionally, as the photo mask alignment and the exposure described above must be performed on both sides of the substrate, that is, as the photo mask alignment and the exposure each have to be performed twice, it is difficult to shorten the fabrication process as long as this method is used.
Usually, each photo mask is placed in intimate contact with the substrate for use, and there is a limit to the number of times the same photo mask can be used. This means that when the photo mask comes to the end of its life, a new photo mask has to be produced; this increases the production cost and is inefficient.
In view of the above problems, it is an object of the present invention to provide a pattern drawing apparatus and pattern drawing method that can reduce the cost and shorten the fabrication process when forming patterns, that have a mirror image relationship to each other with respect to a substrate, on both sides of the substrate. It is also an object of the present invention to provide a test apparatus for use in the pattern drawing apparatus.